4. One‑hundred‑year events or freak wind storms. Microbursts, especially violent thunderstorms with winds over 100 miles per hour, can occur almost anywhere. But when they are associated with long fetches (open areas, such as oceans, where wind is uninhibited by structures, trees, land masses, etc.), they can result in large waves as well as wind. The submarine USS Pampanito in San Francisco suffered $180,000 in hull damage, $250,000 in pier damage, and $200,000 in loss of revenue when it was hit by a "hundred year event" in 1988. Her anchors dragged and a three‑inch synthetic line attached to a tug snapped.
Potential regions of concern: Everywhere, but especially open areas, such as bodies of water, where there are no wind breaks.
5. Seiches. A back‑and‑forth movement in a lake or other land‑locked body of water, varying in duration and resulting in fluctuation of the water level, is known as a seiche. The variation in the water level and the interval between low and high water levels is a complicated function of water depth, bottom profile, wind speed and direction, and barometric pressure. Water level can vary as much as eight to ten feet from one location to another. Cleveland, Ohio experienced an eight‑foot seiche in the early 1990s.
Potential regions of concern: All land‑locked bodies of water, including the Great Lakes region.
6. Earthquakes. The oscillatory and sometimes violent movement of the earth's surface following the release of energy in the earth's crust is called an earthquake. Earthquakes are usually caused by the dislocation of segments of the earth's crust, but they also can be caused by volcanic eruption and man‑made explosives.
a. Tsunamis. Incorrectly called tidal waves (they have nothing to do with tides), tsunamis can be generated by vertical displacement of the earth's crust, volcanic eruption, and submarine landslides. The resultant waves travel about 600 miles per hour, 100 miles or more apart. They are undetectable at sea, but as they approach shallow waters may crest to heights of 100 feet or more and crash inland at speeds of 30 miles per hour. The first wave is usually not the largest, with subsequent waves striking the coast at five to 40‑minute intervals.
About 50 tsunamis have struck the Hawaiian Islands since the early 1800s. At least one tsunami has partially penetrated the Columbia River. Tsunami‑generating earthquakes have occurred all around the Pacific Rim, including Alaska, California, and Hawaii. The Scotch Cap Lighthouse, built in 1903 on a 90‑foot bluff in Alaska, was swept away on April 1, 1946 by a tsunami wave estimated to be 100 feet tall, killing five keepers. The California San Andreas Fault is primarily a horizontal displacement fault and therefore can not cause a tsunami. However, California experienced a tsunami watch, including evacuation of some coastal areas, in October 1994.
Potential regions of concern: Hawaii and Alaska are at high risk; the Pacific coast of Washington, Oregon, and California, is at moderate risk. Tsunamis can also travel up rivers and streams that open to the ocean.